1. Field of the Invention
The present invention relates generally to an apparatus for controlling a continuously variable transmission (hereinafter referred to as "CVT") incorporated in a power transmitting system of an automotive vehicle. More particularly, the invention is concerned with such a control apparatus for controlling the CVT while the vehicle is in a transient running condition.
2. Discussion of the Related Art
A CVT is used for a motor vehicle as a power transmission for controlling the speed ratio e (Nout/Nin, where Nout=output speed of the CVT, Nin=input speed of the CVT) in a continuous fashion, so as to improve the specific fuel consumption of the vehicle. In the CVT, the currently required horsepower is determined as a function of a position .theta. of a throttle valve of the engine. The desired speed Ne of the engine is determined so as to attain the determined horsepower required at each throttle position .theta. with minimum fuel consumption. Conventionally, this desired engine speed is determined, irrespective of whether the vehicle is running in a steady state or in a transient state. Namely, the same desired speed is used for the same throttle position .theta. in both the steady and transient states of the vehicle. In the transient state, it takes a considerably long time until the actual engine speed reaches the determined desired engine speed, which means a relatively low response of the engine to a change in the throttle position .theta.. A solution to obviate this drawback is proposed in Japanese Patent Application No. 58-17550 filed in 1983 in the name of the assignee of the present application, wherein the speed Ne of the engine or the speed ratio e of the CVT is regulated so that the actual engine speed coincides with the desired engine speed No. More specifically, when the throttle position .theta. is changed at a comparatively high rate, the desired engine speed No is changed in steps after a predetermined time delay Ta', by a varying step amount which is equal to a difference (Ns - Ns') multiplied by a predetermined stepping change rate B which is less than 1, where Ns represents a desired steady-state engine speed corresponding to the newly established throttle position .theta., while Ns' represents a desired steady-state engine speed corresponding to the previous throttle position .theta.. Thereafter, the desired engine speed No is gradually changed so that it finally coincides with the desired steady-state engine speed corresponding to the newly established steady-state engine speed Ns. According to this proposed arrangement, the desired engine speed No in a transient vehicle condition is smoothly increased, whereby the actual engine speed is relatively quickly and smoothly accelerated. The delay time Ta' indicated above is provided to detect the final throttle position .theta. determined by the amount of depression of the accelerator pedal by the vehicle driver. However, in the event that the driver depresses the accelerator pedal by a comparatively large amount requiring a high rate of acceleration of the vehicle, which requires a kick-down of the transmission system, for example, the provided delay time Ta' will give the driver an impression or feeling that the vehicle is not highly responsive to the operation of the accelerator pedal. Thus, the delay time Ta' will deteriorate the drivability of the vehicle as felt by the driver. On the other hand, if the depression of the accelerator pedal by the driver is not completed within the determined delay time Ta', the stepping rate of increase of the engine speed is less than the finally desired rate, whereby the vehicle suffers from insufficient acceleration in the initial stage of depression of the acceleration pedal. It is further noted that the manner of controlling the CVT for adequate acceleration of the vehicle varies depending upon the specific running condition of the vehicle at which the accelerator pedal is operated.